Conventionally, a scanning probe microscope (SPM) is used in observation of a minute specimen. An atomic force microscope (AFM) is known as the SPM.
The AFM brings a sharp probe close to the specimen and detects interaction force acting between the probe and the specimen. The cantilever, which has the probe on a tip thereof is used as a force sensor for detecting the interaction force. The AFM performs feedback control of a distance between the probe and the specimen so as to keep the interaction force constant and further scans the probe (or the specimen) in a horizontal direction in a state in which the feedback control is maintained. By this, the probe (or the specimen) goes up and down so as to trace concavity and convexity of the specimen. Then, by recording a trace of scanning, a concavo-convex image of a surface of the specimen is obtained.
A dynamic mode AFM is known as a kind of the general AFM. The dynamic mode AFM vibrates the cantilever with minute amplitude and detects the interaction force between the probe and the specimen as change in frequency, phase, and amplitude of the cantilever vibration. The dynamic mode AFM is utilized not only in a vacuum and an atmosphere but also in a liquid environment.
An acoustic excitation method is used as an excitation method of the cantilever in the dynamic mode AFM. The acoustic excitation method is disclosed in Non-Patent Literatures 1, 2, and 3, for example.
The acoustic excitation method transmits an acoustic wave generated by the piezoelectric vibrator to the cantilever to excite the cantilever. The acoustic excitation method is widely used because of its relatively simple structure. However, there remains a problem of stability and quantitative characteristics in the liquid environment in the acoustic excitation method. Hereinafter, the acoustic excitation method and the problem thereof are described.
The acoustic excitation method generates the acoustic wave by applying AC potential to the piezoelectric vibrator and efficiently propagates the acoustic wave to the cantilever through a medium, thereby exciting the cantilever vibration. In the acoustic excitation method, the acoustic wave propagates through the medium such as a cantilever holder, so that mechanical resonance in a holder structure is excited. In the dynamic mode AFM in liquid, structural resonance propagates in solution in addition to the holder structure to be transmitted to the cantilever. Therefore, there is the problem of complication of response characteristics of the phase and the amplitude of the cantilever vibration, that is to say, frequency response characteristics to a cantilever excitation signal.
Further, a Q-factor of resonance of the cantilever is significantly low in the liquid as compared to that in the vacuum and the atmosphere. Therefore, an effect of the structural resonance increases and this causes deterioration in the stability and reliability of dynamic mode AFM measurement.
In order to solve the above-described problem, it is considered to directly drive the cantilever. As a direct drive method, a magnetic excitation method (Non-Patent Literatures 4 and 5) and a photothermal excitation method (Non-Patent Literature 6) have been suggested. In the magnetic excitation method, magnetic coating is applied to the cantilever or magnetic beads are adhered thereto. In the photothermal excitation method, a gold thin film is provided on the cantilever and infrared light or ultraviolet light is applied thereto.
However, in the direct drive method, a process to coat a back surface of the cantilever with a magnetic body and a metal thin film is necessary. In a case of the AFM in liquid, increase in contamination by elution of a back surface coating material is problematic. Also, in a process of the coating on the back surface, it is difficult to prevent the coating material from going around a surface side on which the probe is formed and there is the problem of deterioration in sharpness of the probe. Further, in order to obtain driving force of the cantilever vibration, a magnetic coil or a laser modulating device is required. Therefore, a device configuration is complicated and general-purpose property thereof is low. By such background, application of the direct drive method is limited as a result.
On the other hand, the acoustic excitation method does not require modification of the cantilever and this may be realized only by a small piezoelectric vibrator and wiring to apply potential. Therefore, the acoustic excitation method is widely utilized in the field of the AFM. Therefore, it is desired to use the acoustic excitation method also when using the dynamic mode AFM in the liquid environment. However, as already described, there is the problem of the complication of the frequency response characteristics of the cantilever by the effect of the structural resonance of the cantilever holder generated by propagation of the acoustic wave in the liquid environment.
Then, it is strongly desired to develop the cantilever excitation method capable of inhibiting the complication of the frequency response characteristics by the configuration as simple as that of the acoustic excitation method. It is considered that, if such cantilever excitation method is provided, the stability and the reliability of the dynamic mode AFM in liquid may be significantly improved by the simple configuration.
The background art is described above in regard to the dynamic mode AFM, especially by taking the dynamic mode AFM used in the liquid environment as an example. However, a similar matter might be problematic in an optional case in which the cantilever is excited in the liquid and the like. That is to say, the similar problem might occur in the cantilever excitation device of optional application.